A Static Var Compensator (SVC) and Static synchronous Compensator (STATCOM) provide reactive power to high-voltage electricity transmission systems and are used for regulating voltage and stabilizing the system.
In an electricity transmission system, denoted power grid in the following, electrical loads both generate and absorb reactive power. The transmission of active power with AC is also associated with reactive power consumption or generation. The load varies considerably during e.g. day and night, and the reactive power balance in the power grid thus also varies. The result can be unacceptable voltage amplitude variations, a voltage depression, or even a voltage collapse. A reactive power compensator can be arranged to continuously provide the reactive power required to control dynamic voltage swings under various power grid conditions and thereby improve the power system transmission and distribution performance. Installing a reactive power compensator at one or more suitable points in the network can increase transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. In addition, the reactive power compensator can mitigate active power oscillations through voltage amplitude modulation.
A dynamic compensator is a reactive power compensator provided with an energy storage. Both active and reactive power support can thereby be supplied. By doing so, voltage variations as well as frequency variations and sudden load changes can be supported.
FIG. 1 illustrates such a dynamic power compensator 1, and in particular a Static synchronous Compensator (STATCOM) comprising a Voltage Source Converter (VSCs) 2. The VSC 2 is on its AC side connected to a power grid 7, typically via a reactor 5 and transformer 6.
The VSC 2 is on its DC side connected to a capacitor bank 3, constituting a DC voltage source. The dynamic power compensator 1 can further be provided with a battery energy storage 4, comprising one or more strings of batteries. The battery energy storage 4 may for example be used in power grids that e.g. require frequency regulation essential for grid stability and short term power support to cover variations in load demand or intermittency in power generation.
The batteries of the battery energy storage 4 are connected in strings with a DC interruptor embodied e.g. by IGBTs, and disconnectors. The battery energy storage 4 may comprise a number of series- and/or parallel-connected battery cells arranged in battery units and several battery units may be series-connected to form a battery string. The battery energy storage 4 may comprise several such strings connected in parallel.
The battery state of charge (SOC) is reduced due to power discharge. For e.g. Li-Ion batteries, the voltage varies with the SOC. The voltage from the battery energy storage 4 may thus vary considerably during a charge-discharge cycle. The reactive power output capability of the dynamic power compensator 1 is dependent on the DC voltage level and will hence be reduced with decreasing SOC. This in turn affects the design of the dynamic power compensator 1 in that the battery energy storage 4 has to be over-dimensioned to be able to handle the voltage variations, e.g. by dimensioning the battery energy storage suitably. This is a costly solution that, for example, requires many components and also a large footprint.